1. Field of the Invention
This invention relates to measurement of pulmonary function in infants and more particularly to producing forced expiratory maneuvers in infants over the entire vital capacity range.
2. Brief Description of the Prior Art
Assessment of pulmonary function is of fundamental importance in the diagnosis and treatment of diseases involving the lung. One of the simplest and yet most useful tests of pulmonary function is the forced expiration. In this test the patient makes a maximum inspiration, filling the lungs to their maximum volume, and immediately expels the inhaled air as rapidly as possible to empty the lungs to the minimum volume attainable by this maneuver. Measurement of the volume rate of expiration and the total amount of air expired over the time of the forced expiration yields a number of parameters that can be used as measures of pulmonary function. Plots of exhaled volume versus time are generated and various measures are derived from the plotted curves. Particularly useful diagnostic data derived from the forced maximum expiratory flow plot are the forced expiratory flow in the first second of the forced expiration (FEV.sub.1), the expiratory flow rate averaged over the period from a volume of 25% to 75% of the total expired air volume (FEV.sub.25-75), and the plot of the flow rate versus volume of expired air, defined as the maximum expiratory flow-volume curve (MEFVC). The data from a maximum forced flow maneuver can also be used in combination with measurements of thoracic gas volume, e.g., obtained using a full-body plethysmograph, nitrogen washout or helium dilution, to determine the proportions of that volume that can and cannot be exchanged by the patient. Such divisions of the total lung capacity (TLC) are generally referred to as fractional lung volumes (FLV). These tests are useful for quantitating levels of dysfunction occurring in relation to both obstructive and restrictive pulmonary processes.
One clinically important application of pulmonary function measurements is found in the evaluation of lung condition and function in adults and older children having cystic fibrosis (CF), wherein maximum expiratory flow-volume curves (MEFVCs) and the fractional lung volumes (FLVs) play a central role. Typically, the presence of obstructive lung disease accompanying cystic fibrosis can be diagnosed by the lower rates of expiratory flow that the patient can produce with maximum effort. When adults and older children having cystic fibrosis are admitted to a hospital for treatment of pulmonary exacerbations with intravenous antibiotics and intensive chest physical therapy, MEFVCs and FLVs are routinely used to monitor changes in lung function from the time of hospital admission to discharge after conclusion of the treatment regimen.
However, infants, i.e., typically those under about two years of age, cannot voluntarily perform the forced expiratory maneuver that is needed to measure forced expiratory flows and to derive measures of fractional lung volumes useful in diagnosis. Hitherto, pulmonary function measurements have been available only for the tidal breathing range in such patients. Such data has some clinical utility, but is not comparable to the data obtained by forced expiratory maneuvers in older patients.
Attempts have been made to obtain forced expiratory volume versus time curves for infants by using mechanical compression of the thorax and abdomen with an inflatable vest or similar mechanical compression device. Such techniques have mostly been limited to forced compression from a condition of end-tidal inspiration. Attempts to extend measurements of respiratory mechanics in infants beyond the tidal breathing range have been limited by the inability to disengage infants from their obligatory tidal respiratory cycle. Recently, Turner, et al., Pediatric Pulmonology 15, 220-224 (1993) have reported a technique that permits rapid thoracic compression (RTC) from an increased lung volume. According to Turner at al., the lung volume of the infants was raised by inflating the infants during tidal inspiration by administering additional air through a face mask at a pressure of about 15 cm H.sub.2 O. An inflatable device was utilized to compress the thorax and abdomen immediately after this enhanced inspiration, while the expired flow and volume were measured as a function of time. However, in this maneuver the infants were not inflated to near TLC and terminated the forced expiratory maneuver prematurely by actively inspiring, even during chest compression. Consequently, although Turner's technique represents an advance over the previous procedures, it still does not permit a forced expiratory maneuver in infants that is similar to the voluntary maneuvers performed by adults and older children, in that it does not permit the generation of forced flows over the entire vital capacity (VC) range.
It is known that infants occasionally pause for a few seconds in their normal tidal respiration, generally after taking a deep breath and exhaling in a kind of sigh. Furthermore, it is known that such a pause can be induced by forcibly enhancing an infant's normal tidal respiration by inflating the infant's lungs synchronously with natural inspiration to a volume greater than normal end-tidal inspiration for a few breaths. However, it has not been known to utilize this induced pause for performing a maximum forced expiratory maneuver in infants.
Accordingly, a need has continued to exist for a method of performing forced expiratory flow maneuvers in infants from lung volumes approaching total lung capacity and extending to residual volume (RV) with flow limitation occurring over most of the VC range.